1. Technical Field of the Invention
This invention generally relates to the field of selectively removing silica from silica containing solutions. More particularly, the invention relates to methods for the selective removal and recovery of silica and silicates from containing brines, preferably without the removal of other ions from the brines. Finally, the invention relates to methods for preventing scale buildup in geothermal power plants and processes employing geothermal brines.
2. Description of the Prior Art
Geothermal brines are of particular interest for a variety of reasons. First, geothermal brines provide a source of power due to the fact that hot geothermal pools are stored at high pressure underground, which when released to atmospheric pressure, can provide a flash-steam. The flash-stream can be used, for example, to run a power plant. Additionally, geothermal brines contain useful elements, which can be recovered and utilized for secondary processes. In some geothermal waters and brines, binary processes can be used to heat a second fluid to provide steam for the generation of electricity without the flashing of the geothermal brine.
It is known that geothermal brines can include various metal ions, particularly alkali and alkaline earth metals, as well as lead, silver and zinc, in varying concentrations, depending upon the source of the brine. Recovery of these metals is potentially important to the chemical and pharmaceutical industries. Typically, the economic recovery of metals from natural brines, which may vary widely in composition, depends not only on the specific concentration of desired the desired metal, but also upon the concentrations of interfering ions, particularly silica, calcium and magnesium, because the presence of the interfering ions will increase recovery costs as additional steps must be taken to remove the interfering ions.
Silica is known to deposit in piping as scale deposits, typically as a result of the cooling of a geothermal brine. Frequently, geothermal brines are near saturation with respect to the silica concentration and upon cooling, deposition occurs because of the lower solubilities at lower temperatures. This is combined with the polymerization of silica and co-precipitation with other species, particularly metals. This is seen in geothermal power stations, and is particularly true for amorphous silica/silicates. Additionally, silica is a known problem in RO desalination plants. Thus, removal of silica from low concentration brines may help to eliminate these scale deposits.
Known methods for the removal of silica from geothermal brines include the use of a geothermal brine clarifier for the removal and recovery of silica solids, that can be precipitated with the use of various seed materials, or the use of compounds that absorb silica, such as magnesium oxide, magnesium hydroxide or magnesium carbonate. In addition to a less than complete recovery of silicon from brines, prior art methods also suffer in that they typically remove ions and compounds other than just silica and silicon containing compounds.
Thus, although conventional methods employed in the processing of ores and brines currently can remove some of the silica present in silica containing solutions and brines, there exists a need to develop methods that are selective for the removal of silica at high yields.